Very simple actual milter.

milter-nomix.py

+## To roll your own milter, create a class that extends Milter.  
+#  See the pymilter project at http://bmsi.com/python/milter.html
+#  based on Sendmail's milter API http://www.milter.org/milter_api/api.html
+#  This code is open-source on the same terms as Python.
+
+## Milter calls methods of your class at milter events.
+## Return REJECT,TEMPFAIL,ACCEPT to short circuit processing for a message.
+## You can also add/del recipients, replacebody, add/del headers, etc.
+
+import Milter
+import time
+import sys
+from Milter.utils import parse_addr
+
+internal_tlds = ["corp", "personal"]
+
+# Determine if a hostname is internal or not. True if internal,
+# False otherwise
+def is_internal(hostname):
+    components = hostname.split(".")
+
+    if components.pop() in internal_tlds:
+        return True
+    else:
+        return False
+
+# Determine if internal and external hosts are mixed based on a list
+# of hostnames
+def are_mixed(hostnames):
+    hostnames_mapped = map(is_internal, hostnames)
+
+    # Num internals
+    num_internal_hosts = hostnames_mapped.count(True)
+
+    # Num externals
+    num_external_hosts = hostnames_mapped.count(False)
+
+    return num_external_hosts >= 1 and num_internal_hosts >= 1
+
+class NoMixMilter(Milter.Base):
+
+  def __init__(self):  # A new instance with each new connection.
+    self.id = Milter.uniqueID()  # Integer incremented with each call.
+
+
+  ##  def envfrom(self,f,*str):
+  @Milter.noreply
+  def envfrom(self, mailfrom, *str):
+    self.mailfrom = mailfrom
+    self.domains = []
+    t = parse_addr(mailfrom)
+    if len(t) > 1:
+      self.domains.append(t[1])
+    else:
+      self.domains.append('local')
+    self.internal = False
+    return Milter.CONTINUE
+
+  ##  def envrcpt(self, to, *str):
+  def envrcpt(self, to, *str):
+    self.R.append(to)
+    t = parse_addr(to)
+    if len(t) > 1:
+      self.domains.append(t[1])
+    else:
+      self.domains.append('local')
+
+    if are_mixed(self.domains):
+      # FIXME: log recipients collected in self.mailfrom and self.R
+      self.setreply('550','5.7.1','Mixing internal and external TLDs')
+      return Milter.REJECT
+        
+    return Milter.CONTINUE
+    
+def main():
+  socketname = "/var/run/nomixsock"
+  timeout = 600
+  # Register to have the Milter factory create instances of your class:
+  Milter.factory = NoMixMilter
+  print "%s milter startup" % time.strftime('%Y%b%d %H:%M:%S')
+  sys.stdout.flush()
+  Milter.runmilter("nomixfilter",socketname,timeout)
+  logq.put(None)
+  bt.join()
+  print "%s nomix milter shutdown" % time.strftime('%Y%b%d %H:%M:%S')
+
+if __name__ == "__main__":
+  main()